Piezoelectric transducers have been conventionally used to convert electric signals into sound waves or other mechanical vibrations, or to convert mechanical vibrations into electric signals. They convert electric signals into mechanical vibrations or vice versa by utilizing the morphological change of a crystal which occurs on voltage application, or conversely by monitoring the voltage generated by a pressure applied on a crystal.
As an example of piezoelectric transducer, a probe in an ultrasonic diagnostic equipment is well known. Such a probe is taught in Ide, M.: Recent medical applications of ultrasonic waves; the Journal of Acoustic Society of Japan, Vol. 33, No. 10, 1977, pp. 586-591 (in Japanese), and in Ide, M.: Recent progress in ultrasonic diagnostic apparatus; the Journal of Acoustic Society of Japan, Vol. 36, No. 11, 1980, pp. 576-580 (in Japanese). The former describes in detail the scanning systems for linear, arc-shaped, circular, sector, radial and other ultrasonic beams while the latter explains the principle of the electronic linear scanning method which is recently used quite widely, the structure of an actual electronic linear scanning probe, and the principle of deflection of ultrasonic beams caused by the phase delay.
The probe for the linear scanning method, however, is defective in that radiated ultrasonic beams focus linearly. Focusing on a spot is most desirable to obtain images with high positional precision. In order to focus ultrasonic beams, it is desirable to have a sound source which has a curved surface, especially a spherical surface.
This apparatus has a patent application for a piezoelectric transducer in which the sound source has a curved surface (JPA laid-open Sho 60-111600, referred to herein as the Application '600). The specification and drawings of this application '600 show an embodiment of a piezoelectric transducer with a curved surface which is formed on a curved base, and describe sound radiation and focusing. However, the device in the application '600 is not intended to be used as a probe, and therefore does not consider the focus control of beams.
In order to control the convergent point of radiated beams by the device of application '600, a method is conceivable wherein ring-shaped electrodes are arranged concentrically and formed into plural piezoelectric transducer elements, and driving pulses which are applied to each of the respective elements are sequentially delayed. But this method is also defective because when driving pulses are fed to an arbitrary electrode, two things happen. First the driven section vibrates due to the expansion/contraction caused by piezoelectric effect.fwdarw.the vibration is transmitted to an adjacent piezoelectric transducer element, and voltage signals are generated on the electrodes of the element due to its piezoelectric characteristics.fwdarw.vibration is thus further transmitted to an element adjacent thereto. Second, an electric field is generated inside a piezoelectric transducer element due to the supplied driving pulses.fwdarw.the electric field leaks to another element adjacent thereto to drive it, or an electric voltage is apparently generated between electrodes of the element. When it is used as a probe, sound waves excited by electric driving pulses are radiated at a target (e.g. bio tissues) and the sound waves reflected therefrom are received and converted into electric signals by using a single element. Therefore, if vibration or voltage is leaked to other elements, the state becomes similar to when ultrasonic signals are inputted from outside to cause noise.
This invention was conceived to solve such problems as encountered in the prior art and aims to provide a piezoelectric transducer which can generate mechanical vibrations focusing substantially on one point (a convergent point) and which can control such convergent point.